In numerous analytical, scientific, and technical applications, it is necessary to determine the amount of an isotope in a material. Isotopes are different forms of the same chemical element, having atomic nuclei of different masses. For example, naturally occurring carbon consists predominantly of 12C, i.e., carbon having atomic mass of 12 atomic mass units with small amounts of 13C and 14C isotopes, having atomic masses of 13 and 14 atomic mass units respectively. For example, in medical testing and in drug development, it is a common practice to administer a drug or other substance to a living subject which has molecules incorporating 13C or 14C as a “tracer,” and then measure the amount of 13C or 14C which appears in a particular tissue of the subject, or in substances exhaled or excreted by the subject. Where carbon is incorporated in a moiety such as small multi-atomic ion, radical, or molecule such as carbon dioxide (CO2), moieties containing different isotopes of carbon will absorb and emit light at different wavelengths. This phenomenon is related to differences in the rotational and vibrational states of the multi-atomic moieties which arise from the different masses of carbon atoms.
Spectroscopy can be used to measure the amounts or relative concentrations of particular isotope-bearing moieties present in a sample. In conventional absorption spectroscopy, light at a wavelength corresponding to an absorption wavelength of a moiety containing a particular isotope is directed through a sample. The intensity of light after passage through the sample is measured and compared with the intensity of the applied light. However, conventional absorption spectroscopy requires a large sample containing a substantial concentration of the moiety to be measured. It is a relatively insensitive test.
As described, for example, in Baev, V. M., Latz, T., and Toshek, P. E., “Laser Intracavity Absorption Spectroscopy,” App. Phys., V. 69(3), 171-202 (1999), it has been suggested that the sensitivity of absorption spectroscopy can be increased by placing the sample cell containing the sample inside of a laser. For example, a gas laser includes a tube filled with a gas disposed in a cavity between a pair of mirrors, one of which is a partially reflective mirror. When energy is applied to the gas, light emitted by the gas passes back and forth between the mirrors. As the light impinges on the medium, it stimulates emission of additional light which is coherent with the impinging light. A small portion of the light passing back and forth between the mirrors passes out of the laser through the partially reflective mirror. Thus, if the cell containing the sample is placed between the mirrors, it is exposed to each photon of light many times as the photon passes back and forth between the mirrors. Systems of this type are commonly referred to as “intracavity laser absorption spectroscopy” or “ICLAS”. ICLAS typically is on the order of 100 times more sensitive than conventional absorption spectroscopy.
My own U.S. Pat. Nos. 5,394,236 and 5,818,580 describe techniques in which light is directed through a sample which includes multi-atomic moieties such as molecules or ions containing particular isotopes. The analyte typically is maintained in a condition such that at least some of the isotope-bearing moieties in the analyte are in excited states. For example, the analyte may be maintained in this condition by maintaining it as an ionized gas or plasma. At least some of the electrons in the molecules or ions are at energy levels higher than the energy levels occupied in the ground or normal state of the moieties. Such excited states have transition energies corresponding to the energy released upon transition from the excited state to a lower energy state, or absorbed upon the reverse transition, from the excited state to another, higher energy state. The transition energies are different for moieties incorporating different isotopes, as for example, 12CO2, 13CO2, and 14CO2. Light at a wavelength corresponding to a transition energy of excited moieties including one isotope will interact with the moieties including that isotope, but will not substantially interact with moieties including the other isotopes. Light at the appropriate wavelength for each isotope to be monitored is applied to the sample, as by directing one or more beams from one or more lasers onto the sample and monitoring an induced effect, most commonly the optogalvanic effect. The optogalvanic effect refers to a change in the electrical impedance of a plasma. Methods according to the aforementioned patents provide considerable improvement over absorption spectroscopy. Methods according to these patents have been adopted, for example, in monitoring of environmental conditions and in certain medical testing procedures.
Murnick, D. E., Perez, M. A., and Polickal, M., “Laser Based 14C Atom Counting For Biomedical Studies,” Proceedings of the International Isotope Society Conference, June 2003, characterizes certain methods of this type as having a sensitivity “exceeding three picomoles of 14CO2,” i.e., as being capable of detecting quantities of CO2 on the order of 3×10−12 moles of CO2. A “mole” is a quantity of a substance which contains 6.02×1023 molecules or atoms, and which has a mass in grams equal to the atomic or molecular weight of the substance in atomic mass units (“AMU”). For example, a mole of 14CO2 (total molecular weight 48 AMU) corresponds to 48 grams of 14CO2. Thus, 3 picomoles is about 1.4×10−10 grams of CO2, i.e., about one tenth of 1 billionth (10−9) of a gram. The same article notes that “a laser can be configured to incorporate a sample cell inside the laser cavity, which should yield a gain factor of 400 due to the circulating laser power,” i.e., that an arrangement similar to that used in ICLAS can be employed with optogalvanic spectroscopy, and that “the proposed intracavity optogalvanic effect system should then yield sensitivity of order 10−10 to 10−11, and may provide even better results.” A table in the same article states that such methods might be projected to have a sensitivity on the order of “10−12 of better,” i.e., one part 14C in 1012 parts 12C and 13C, using a sample size of 1 micromole. This corresponds to detection of 10−18 moles of 14CO2, i.e., about 6×105 or 600,000 molecules of 14CO2. While such a test is indeed sensitive, still further improvement would be desirable. For example, where a small dose of an isotopically-labelled drug is administered to a subject such as a human or animal, metabolic studies may require detection of the isotope at levels on the order of 10−20 moles (about 6000 molecules) of 14CO2 or better, using a small sample size. Until the present invention, the only test method capable of achieving this level of sensitivity has been accelerator mass spectroscopy (“AMS”). Because AMS at this level of sensitivity requires a costly ion accelerator, it is a very expensive test method. Moreover, AMS suffers from other drawbacks.